This invention relates to barrier-coated polyesters, preferably barrier coated polyethylene terephthalate (PET) and articles made therefrom. Preferably the barrier-coated PET takes the form of preforms having at least one layer of a barrier material and the bottles blow-molded therefrom. This invention further relates to methods of making articles formed of barrier coated polyester.
The use of plastic containers as a replacement for glass or metal containers in the packaging of beverages has become increasingly popular. The advantages of plastic packaging include lighter weight, decreased breakage as compared to glass, and potentially lower costs. The most common plastic used in making beverage containers today is PET. Virgin PET has been approved by the FDA for use in contact with foodstuffs. Containers made of PET are transparent, thin-walled, lightweight, and have the ability to maintain their shape by withstanding the force exerted on the walls of the container by pressurized contents, such as carbonated beverages. PET resins are also fairly inexpensive and easy to process.
Despite these advantages and its widespread use, there is a serious downside to the use of PET in thin-walled beverage containers: permeability to gases such as carbon dioxide and oxygen. These problems are of particular importance when the bottle is small. In a small bottle, the ratio of surface area to volume is large which allows for a large surface for the gas contained within to diffuse through the walls of the bottle. The permeability of PET bottles results in soft drinks that go “flat” due to the egress of carbon dioxide, as well as beverages that have their flavor spoiled due to the ingress of oxygen. Because of these problems, PET bottles are not suitable for all uses desired by industry, and for many of the existing uses, the shelf-life of liquids packaged in PET bottles is shorter than desired.
Although the plastic beverage container industry is large and competitive and the permeability problem with PET containers has been known since the inception of their use, there still is no good working solution to the permeability problem. Attempts to produce containers with barrier coatings have been heretofore largely unsuccessful.
Most of the problem with producing coated containers comes from the difficulty in finding suitable barrier materials. When most materials are placed on PET they will not adhere at all or they will adhere so weakly that they will delaminate from the PET over a short period of time or under minimal stress. Examples of such materials are polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC). Materials that do adhere to PET often do not have good barrier properties or have other characteristics that do not make them suitable for use in a low-cost commercial barrier coated container.
U.S. Pat. No. 5,464,106 to Slat, et al, describes bottles formed from the blow molding of preforms having a barrier layer. The barrier materials disclosed are polyethylene naphthalate, saran, ethylene vinyl alcohol copolymers or acrylonitrile copolymers. In Slat's technique, the barrier material and the material to form the inner wall of the preform are coextruded in the shape of a tube. This tube is then cut into lengths corresponding to the length of the preform, and is then placed inside a mold wherein the outer layer of the preform is injected over the tube to form the finished preform. The preform may then be blow-molded to form a bottle. The drawbacks of this method are that most of the barrier materials disclosed do not adhere well to PET, and that the process itself is rather cumbersome.
A family of materials with good barrier characteristics are those disclosed in U.S. Pat. No. 4,578,295 to Jabarin. Such barrier materials include copolymers of terephthalic acid and isophthalic acid with ethylene glycol and at least one diol. This type of material is commercially available as B-010 from Mitsui Petrochemical Ind. Ltd. (Japan). These barrier materials are miscible with polyethylene terephthalate and form blends of 80-90% PET and 10-20% of the copolyester from which barrier containers are formed. The containers made from these blends are about 20-40% better gas barriers to CO2 transmission than PET alone. Although some have claimed that this polyester adheres to PET without delamination, the only preforms or containers disclosed were made with blends of these materials. There is no evidence that anyone heretofore has actually made a laminar preform or container using these materials from which to base such a statement.
Another group of materials, the polyamine-polyepoxides, have been proposed for use as a gas-barrier coating. These materials can be used to form a barrier coating on polypropylene or surface-treated PET, as described in U.S. Pat. No. 5,489,455 to Nugent, Jr. et al. These materials commonly come as a solvent or aqueous based thermosetting composition and are generally spray coated onto a container and then heat-cured to form the finished barrier coating. Being thermosets, these materials are not conducive to use as preform coatings, because once the coating has been cured, it can no longer be softened by heating and thus cannot be blow molded, as opposed to thermoplastic materials which can be softened at any time after application.
Another type of barrier-coating, that disclosed in U.S. Pat. No. 5,472,753 to Farha, relies upon the use of a copolyester to effect adherence between PET and the barrier material. Farha describes two types of laminates, a three-ply and a two-ply. In the three-ply laminate, an amorphous, thermoplastic copolyester is placed between the barrier layer of phenoxy-type thermoplastic and the layer of PET to serve as a tie layer to bind the inner and outer layers. In the two-ply laminate, the phenoxy-type thermoplastic is first blended with the amorphous, thermoplastic copolyester and this blend is then applied to the PET to form a barrier. These laminates are made either by extrusion or by injection molding wherein each layer is allowed to cool before the other layer of material is injected.
Thus, the need for barrier-coated PET preforms and containers which are economical, cosmetically appealing, easy to produce, and have good barrier and physical properties remains unfulfilled.